Add controls to set limits on the x and y axes

This adds control about the plot limits.

Currently implemented as 4 number inputs for min, max for x and y.
This works fine, except that one needs to remember to unset them if you change the data to plot.

flot has some 'zoom' support, using a rectangle to select an area to zoom in to.
However, it does seem to have a nice reset option.. otherwise it would be ideal..